Plastics and Static Electricity

I’m not sure if anyone does this anymore but when I was a kid it was great fun to shuffle your feet on carpeting and then zap your unsuspecting sibling with a nice static shock. This typically worked best if you had nylon carpeting and it was nice and dry in the house like in the winter time.

This worked because plastic is a great insulator and thus cannot dissipate static electricity very well. When we need to reduce the likelihood of static build up in plastics, we have some options. There are materials described as having anti-stat additives and other materials described as being statically dissipative and still others as being conductive. What’s the difference?

There are multiple reasons why one might want to reduce static electricity build up on a plastic part. Parts that have static charges can attract dust and in extreme situations, can cause explosion. In film manufacturing, static charge can cause sheets of film to stick together. Static charge can also cause damage to electronic components that might come into contact with a plastic housing or tray.

Measuring a plastic materials ability to dissipate static electricity is done with a test called surface resistivity. It sounds complex but it is really just using an ohm-meter to measure resistance across the surface of a plastic plaque. Surface resistivity is measured in ohm/sq. If you want to understand this measurement better read this: http://www.esdjournal.com/techpapr/ohmmtr/ohm.htm

What’s important to note is that the higher the resistance number the less conductive a material is. Here are some surface resistivity numbers for plastics:

10¹² or 1,000,000,000,000 ohm/sq or higher
Plastic that is insulating or with virtually no ability to dissipate static electricity

10¹⁰ – 10¹² or 10,000,000,000 to 1,000,000,000,000 ohm/sq
Materials in this range are considered to have anti-static properties

10⁶ – 10¹⁰ or 1,000,000 to 10,000,000,000 ohm/sq
Materials in this range are considered to have static dissipation properties

10-10⁶ of 10 to 1,000,000 ohm/sq
Materials in this range are considered to be conductive

Insulating plastic is easy to understand but here are the differences between Anti-Static, Static Dissipative and Conductive materials:

Anti-Static

Anti-static materials have additives that cause what are called hydrophilic molecules to continually rise to the surface of the material. If you have seen the commercials for Rust-oleum Never Wet (http://www.neverwet.com/) you have seen a hydrophobic molecule. Hydrophobic molecules absolutely hate water and cause water molecules to be repelled from its surface. Hydrophilic molecules are the opposite. They love water and attract it to the surface. In the case of anti-static additives, the hydrophilic molecules attract water molecules to the surface of the part. Water is not actually a good conductor but water typically contains a lot of minerals which make it a great conductor. The small amount moisture on the surface will prevent a static charge from building up on the surface. It is not enough moisture for you to notice. The part will not feel moist in any way.

It typically takes some time for the anti-stat additive to come to the surface of the part. Usually about 2 days. In addition, it does not last forever, typically 1 to 2 months. The anti-static properties require some amount of relative humidity and will work better when the relative humidity is higher.

Anti-static additives are not expensive. Grades of polypropylene and polyethylene with anti-static additives might cost a penny or two more per pound than those without. You can also buy anti-stat in a concentrate form that can be blended into some materials. Manufacturers will sometimes refer to these concentrates as master batches.

Static Dissipation

Plastics with static dissipative properties are different in that they have conductive fillers in them that actually make the plastic conduct a small amount of electricity. These fillers tend to be things like carbon black or carbon powder among other things.

The primary difference between this and anti-static material is that if a statically charged item comes into contact with a static dissipative plastic part, the plastic part will take the static charge out of the item and dissipate it away. For instance this might be useful for electronics assembly. A tray used to hold sensitive electronics made of statically dissipative materials will help to dissipate any static charge from a worker so that it does not damage the electronics. The tray would need to be connected to a ground strap or on a metal work bench or conveyor to full do it’s job.

Unlike anti-stat additives, the fillers used in this type of material start working immediately and will work for the life of the part.

Static dissipative materials are very expensive. The fillers are expensive and difficult to work with. Compounders that make these types of materials have to be very careful in dealing with fine conductive powders. If these powders get in the air, they can cause explosions and cause electrical problems in plants if they get into electrical junction boxes. The finished material has the filler encapsulated in the material so risks to processors are minimal.

Conductive

Conductive materials have higher amounts of conductive fillers and also more conductive fillers like carbon fiber or stainless steel fiber. Conductive plastic will conduct large amounts of electricity. This will provide extreme amounts of static dissipation and can also be used for shielding electronics from electromagnetic interference or EMI. These are typically overkill for static electricity problems but are sometimes needed.
Conductive plastics are also very expensive but sometimes they can be used to replace metal which can save money and weight.

Slip Agents

It is worth mentioning that adding a slip agent which is nothing more than a lubricant to plastic material can have some impact on reducing static charge build up. The slicker surface of parts lowers friction and thus reduces the likelihood of static build up. Slip agents are very inexpensive and can be added in a concentrate form to polypropylene and polyethylene. It also has the benefit of acting as a mold release and will enhance the flow rate of the material.

Surface Sprays and Anti-Static Cleaners

There are also anti static sprays and cleaners that can be used to reduce static build up on plastic parts. These sprays will not dissipate static electricity but they can be an effective way to reduce static build up if adding additives to the plastic is not an option.

Conclusion

Most materials with anti static additives do not actually show surface resistivity on the data sheets but static dissipative and conductive material data sheets typically will. Often, part prints that require static dissipation or conductivity will call out a range for the material to meet which makes it easier to choose a material. It you are unsure about the requirements, you might want to contact you friendly neighborhood material manufacturer for some advice on what to use. Going overkill with static dissipation will cost a lot of money so you will want to determine what you really need.